The invention relates to locating control signals in a mobile communications system.
Mobile communications systems, such as cellular or personal communications services (PCS) systems, are made up of a plurality of cells. Each cell provides a radio communications center in which a mobile unit establishes a call with another mobile unit or a wireline unit connected to a public switched telephone network (PSTN). Each cell includes a radio base station, with each base station connected to a mobile switching center that controls processing of calls between or among mobile units or mobile units and PSTN units.
From the original advanced mobile phone system (AMPS) standard, additional wireless protocols have been developed and implemented. One such protocol is the time-division multiple access (TDMA) protocol, originally implemented as the IS-54 standard (EIA/TIA/IS-54) and later followed by the IS-136 standard (TIA/EIA-136) from the Telecommunications Industry Association (TIA). With IS-136 TDMA, each channel carries a frame that is divided into six time slots to support up to three mobile units per channel. If half-rate voice coders are used, then each channel may be time shared among up to six mobile units. Other TDMA-based systems include Global System for Mobile (GSM) communications systems, which use a TDMA frame divided into eight time slots (or burst periods).
Traditional speech-oriented wireless systems, such as the IS-136 and GSM TDMA systems, utilize circuit-switched connection paths in which a line is occupied for the duration of the connection between a mobile unit and the mobile switching center. Such a connection is optimum for communications that are relatively continuous, such as speech. However, data networks such as local area networks (LANs), wide area networks (WANs), and the Internet use packet-switched connections, in which communication between nodes on a communications link is by data packets. Each node occupies the communications link only for as long as the node needs to send or receive data packets. With the rapid increase in the number of cellular subscribers in conjunction with the rising popularity of communications over data networks such as intranets or the Internet, a packet-switched wireless data connection that provides access to the data networks, electronic mail, files in databases, and other types of data has become increasingly desirable.
Several packet-based wireless connection protocols have been proposed to provide more efficient connections between a mobile unit and a data network. One such protocol is the General Packet Radio Service (GPRS) protocol, which complements existing GSM systems. Another technology that builds upon GPRS that has been proposed is the Enhanced Data Rate for Global Evolution (EDGE) technology, which offers a maximum data rate of 384 kilobits per second (kbps) and complements both GSM and IS-136 TDMA systems.
One technique to provide for packet data communications is to overlay a packet-switched system with existing circuit-switched mobile communications systems, such as the IS-136 and GSM systems. However, with the overlay solution, a mobile unit may need to acquire control channels in both the packet-switched system and circuit-switched system when the mobile unit is initialized, such as when the mobile unit is first started or when it initially enters into an area controlled by a mobile switching center. For example, the mobile unit may have to search through many traffic and other control channels to find a desired control channel. Such a process may be time consuming, during which the mobile unit is unavailable for use. A need thus exists to provide an improved mechanism to locate control channels in a mobile communications system that includes a plurality of links, such as a packet-switched link and a circuit-switched link.
In general, according to one embodiment, a method for use in a mobile communications system having a first link and a second link includes communicating signals with a series of frame structures in the first link. Information indicating a current frame structure of the first link is identified, and a control signal in the second link is located using the information indicating the current frame structures of the first link.
Some embodiments of the invention may include one or more of the following advantages. A mechanism is provided to allow a mobile unit to more efficiently and quickly locate a desired control signal, such as a synchronization burst, in a mobile communications system having two or more links (e.g., a packet-switched link and a circuit-switched link). Such a mechanism is especially advantageous when used in a system in which at least one of the links communicates certain control channels (such as synchronization channels) as bursts rather than as a beacon that is transmitted continuously. User wait time during mobile unit initialization may be reduced since the mobile unit can more quickly locate and acquire a desired control signal.
Other features and advantages will become apparent from the following description and from the claims.